1. Field of the Invention
The invention relates to a method and apparatus for pressure infiltration casting.
2. Description of the Prior Art
In currently used pressure infiltration processes, as described by U.S. Pat. Nos. 5,111,870 and 5,111,871 to Cook and in general reviews of the state of the pressure infiltration casting art such as Cook et al., "Pressure infiltration casting of metal matrix composites", Materials Science and Engineering, A144, (1991) pages 189-206, a cold mold containing a preform is loaded into a combined pressure vessel/vacuum furnace. A charge of solid infiltrant which can be metal is placed on top of the preform and is separated from the preform by a filter. The filter is characterized by sufficiently low permeability and lack of wetting with the liquid infiltrant to prevent premature infiltrant penetration into the preform and chemical inertness with respect to the infiltrant to avoid contamination of the infiltrant. The filter material also acts as a thermal insulator so that infiltrant charge temperature and preform temperature can be independently controlled.
Next, the preform is heated and the solid infiltrant charge is melted under vacuum in the pressure vessel/vacuum furnace. Since the infiltrant is melted in a vacuum and the mold is not gas permeable, a vacuum is isolated in the preform contained in the mold cavity.
Then, the pressure vessel/vacuum furnace is pressurized to create a pressure gradient between the pressurized mold exterior and the vacuum isolated in the preform contained within the mold interior. It is this pressure differential that drives the infiltration process.
After infiltration is complete, the final step of the process is solidification of the infiltrated preform. Solidification of the infiltrated preform is also conducted within the pressure vessel/vacuum furnace by providing a temperature gradient appropriate to result in directional solidification. Several techniques to obtain directional solidification in a pressure infiltration process are known in the art including lowering of the infiltrated preform into a "chill zone" as demonstrated by Klier et al., "Fabrication of cast particle-reinforced metals via pressure infiltration", Journal of Materials Science, 26, (1991), pages 2519-2526 or, alternatively, lifting a cooled chill device to contact the preform as described in U.S. Pat. Nos. 5,111,870 and 5,111,871 to Cook. During directional solidification in a pressure infiltration process, liquid infiltrant in the hot zone of the infiltrated preform solidifies last and serves as a sprue and reservoir for feeding porosity as the rest of the infiltrated preform solidifies.
The three steps involved in the foregoing prior art pressure infiltration processing step (1) of preform and infiltrant charge heating and evacuation step (2) of preform infiltration, and step (3) of infiltrated preform solidification, each take different amounts of time. Preform and infiltrant charge heating and evacuation take the longest amount of time, infiltrated preform solidification takes less time than preform and infiltrant charge heating and evacuation and pressure infiltration of the preform takes the least time. For example, using a two inch by four inch by eight inch mold cavity, 600 grams of aluminum infiltrant and a silicon carbide particulate preform as would typically be encountered in use of prior art pressure infiltration methods, in the range of from about 2 to about 3 hours are needed to preheat the preform and melt the aluminum infiltrant charge under vacuum, less than about 1 minute is required to infiltrate the heated preform with the molten aluminum infiltrant and less than about 6 minutes are needed to cool the mold to a temperature less than the solids temperature of the aluminum infiltrant. Once the mold is removed from the pressure vessel/vacuum furnace, the pressure vessel/vacuum furnace can be used to resume the three step pressure infiltration process.
While the foregoing prior art pressure infiltration process is a highly effective and controllable process, the throughput of finished pressure infiltrated articles is inherently limited by the slowest step of the pressure infiltration process, that of heating the preform and melting the infiltrant charge, which as demonstrated by the foregoing example, is as long as 3 hours by comparison with a total of at most 16 minutes for the other two steps of the process, preform infiltration and infiltrated preform solidification combined. Although the pressure vessel/vacuum furnace pressure infiltration capability is only needed during the two shortest steps of the pressure infiltration process, this tooling is in constant use, even for the most time consuming steps of the process, because it is also used for preform and infiltrant heating and evacuation.
Thus, according to existing pressure infiltration techniques, preform and infiltrant heating and evacuation as well as pressure infiltration are performed sequentially in the same pressure vessel/vacuum furnace tooling, thus occupying this multipurpose tooling for all three stages of the pressure infiltration casting process, when, in fact, the pressure vessel function of the tooling is only required for the rapidly accomplished step of pressure infiltration and solidification. These existing pressure infiltration processes are limited by their slowest step, preform and infiltrant heating.
Thus, there exists a need for a rapid and economical pressure infiltration process wherein the throughput of finished articles is limited only by the solidification rate of the infiltrated mold cavity and wherein the steps of mold cavity and infiltrant heating and evacuation are performed in separate apparatus from the steps of mold cavity infiltration and infiltrated mold cavity solidification.